Kaidan: RWD Utility.

Kaidan is a simple script that uses a file as input for the url's and takes a screenshot of every page using the different resolutions.

Install phantomjs and casperjs.Copy kaidan.js and urls.txt in a folder, open urls.txt and add the pages you want to test, separated in a new line.
http://www.lastwebdesigner.com
http://www.antoniofullone.it
Then open the terminal go to the folder with kaidan and run the script using casperjs kaidan.js. It will save all the screenshots in a folder and name each image in this : page_title_width.png The resolutions are managed in an array that can be modified depending on your needs.
var viewports = [ {w:320,h:480}, {w:480,h:640}, {w:800,h:600}, {w:1024,h:768}, {w:1440,h:960}, {w:1920,h:1080} ]